1. Field of the Invention
The present invention relates generally to aqueous coating compositions comprising functional group-containing crosslinkable resins (the "resin component") and, optionally, a curing/crosslinking agent for functional groups of the crosslinkable resin.
2. Background
Most successful coating compositions, particularly those suited for use in the automotive industry, have been based upon generally lipophilic crosslinkable binder resins diluted in organic solvents. Increasingly stringent health/safety and environmental legislation, however, is making the use of organic solvents an expensive and unfashionable option for the coatings industry. As such, the industry is intensively seeking waterborne alternatives to their long-standing and commercially successful organic solvent based products; however, it has been difficult at best to make the switch to water without significant loss of coating stability and/or resulting film quality.
One option pursued has been to render the lipophilic resins water-dilutable (so called "self-emulsifying" resins). This can and has been done, for example, by building hydrophilic functionality into a polymer backbone or by providing the polymer with a structure wherein the hydrophilic functionality is made pendant to such polymeric backbone. See, for example, U.S. Pat. Nos. 2,634,245, 3,379,548, 4,116,902, 4,133,786, 4,028,313, 4,255,541, 4,277,380, 4,303,565, 4,315,044, 4,321,305, 4,333,865, 4,585,828, 4,608,406 and 5,021,544, British Patents GB 1 117 126, GB 1 199 810, GB 1 517 767 and WO 83/00151, EP-A-157291, EP-A-0355892, EP-A-0445863 and EP-A-0469646. The results obtained with this option, however, are not always satisfactory because the resins often possess a relatively large number of hydrophilic groups, tending to render the resulting films somewhat water-sensitive.
Another option has been to disperse such lipophilic resins in water with the aid of separate dispersing agents. For example, U.S. Pat. Nos. 4,352,898; 4,598,108; 4,737,530; 4,929,661; and EP-A-0262720 disclose the use of relatively low molecular weight organic compounds as dispersing agents. The aqueous coatings described, while satisfactory in many respects, may still require the use of significant amounts of organic co-solvents. Additionally, the described dispersants may evaporate and contribute to the overall volatile organic compounds (VOC), and what dispersant remains in the resulting film tends to act as an undesirable hydrophilic center.
Relatively higher molecular weight surfactants have also been used. For example, U.S. Pat. Nos. 3,945,964, 4,318,832 and W084/00169 describe the use of various non-ionic (e.g., polyoxyalkylene) and anionic surfactants. The described surfactants, however, are generally non-reactive and, while solving certain disadvantages of the lower molecular weight organic compounds, have a stronger tendency than the lower molecular weight counterparts to remain as undesirable hydrophilic centers in the resulting films.
In still further attempts to overcome the problems associated with the above-referenced systems, it has been proposed to use surfactants which become bound into the film structure during the crosslinking reaction. Such "reactive emulsifiers" have been described, for example, in Australian Patent No. AU-A-82247/87, British Patent Nos. GB1517767, GB-A-2100271 and GB-A-2112793, German Patent No. DE-OS-2455896, Canadian Patent No. CA-A-2007097, U.S. Pat. Nos. 3,979,346; 4,028,313; 4,221,685; 4,233,194; 4,321,170; 4,331,573; 4,446,258; 4,538,000 and European Patent No. EP-A-0355892.